This invention relates to a device for extracting from a composite synchronizing signal a synchronizing signal whose constituent synchronizing pulse has a desired cycle.
In a television receiver, a composite synchronizing signal obtained, through a synchronizing signal separation circuit, from a video signal includes a horizontal synchronizing pulse, vertical synchronizing pulse and equalizing pulse. Speaking of a horizontal scanning, a horizontal oscillation circuit is provided to obtain a horizontal scanning pulse and it is subject to an automatic frequency control (A.F.C) by both a horizontal synchronizing pulse and the output of a horizontal scanning pulse deflection device and a horizontal scanning is effected using the horizontal scanning pulse. In the case of such horizontal scanning it is unnecessary to separate a horizontal synchronizing signal from a composite synchronizing signal. However, there has been proposed a horizontal deflecing means adopting the method of obtaining a horizontal scanning pulse, without using such horizontal oscillation circuit, by frequency-dividing the output of an oscillator (for example, an oscillator for a color sub-carrier)-which is naturally incorporated into a TV receiver- having an oscillation frequency of an integral multiple of the frequency of a horizontal synchronizing pulse, and drawing the phase of the horizontal scanning pulse into the phase of the horizontal synchronizing pulse. Where such horizontal deflection means is adopted, it is necessary to extract, from a composite synchronizing signal separated from a video signal, a horizontal synchronizing signal whose constituent synchronizing pulse is a horizontal synchronizing pulse. However, the composite snychronizing signal sometimes includes a noise pulse in addition to the equalizing pulse and horizontal and vertical synchronizing pulses and, therefore, when the phase of the horizontal scanning pulse obtained by the above-mentioned frequency division is drawn into a pulse -- a pulse regarded as a horizontal synchronizing pulse -- extracted from a composite synchronizing signal using means for detecting a pulse width only, it is sometimes impossible to effect a satisfactory scanning.
In a PCM (Pulse Code Modulation) communication, a plurality of sets of synchronizing signals for address are included in a pulse code modulated wave and it is desired that a desired synchronizing signal consisting of synchronizing pulses having a predetermined cycle be extracted from the pulse code modulated wave.
It is accordingly the object of this invention to provide a device for extracting from a composite synchronizing signal, without being influenced by noises, a synchronizing signal whose constituent synchronizing pulses have a predetermined cycle.